The heat pump
The gas boiler is 19 years old, heat-only, and sits in a garage we're converting into an office, so it has had a good innings. In December 2026 an Octopus Cosy 9 replaces it, the gas meter comes out, and the house goes fully electric. These are the sums we did beforehand, including the extra electricity the heat pump uses, which a lot of savings estimates leave out.
Everything on this page is modelled, not measured. The install hasn't happened yet. Heat demand comes from our own historic gas meter readings, and the efficiency figure is the manufacturer's at the quoted flow temperature. Unlike the stats, none of it has been measured. After the first winter we'll replace it with real numbers, and you'll be able to see how far out we were.
What we pay
£4,375after the £7,500 grant
Heat needed
15MWh/yrspace + hot water
Gas bill removed
£1,427at today's 7.39p gas
Actual saving
£838/yrafter the new electricity
Which gas price?
The gas price makes more difference than anything else, so we'll start there. We're fixed at 6.2382p/kWh until March 2027, and the market average is already 7.39p. A cheap fix that ends in a few months would make the boiler look better than it is, so the main figures on this page all use the market price. The fixed-price figures are shown alongside for reference.
At our fixed 6.2382p · until Mar 2027
£634/yrGas bill £1,223. Payback 6.9 years on the raw outlay.
At the market 7.39p · what we'll actually face
£838/yrGas bill £1,427. Payback 5.2 years on the raw outlay.
The extra electricity
Getting rid of a £1,427 gas bill doesn't save £1,427. The heat pump makes the same heat from electricity, which is 4,545 kWh of new demand, most of it in the four coldest months. Once that's paid for, well over half the gas saving has gone.
Keep the gas boiler
Heat pump, no gas at all
That leaves £838 a year, which is still a saving but a lot less than the brochure figure. If someone quotes you a heat pump saving close to your whole gas bill, they've probably left out the electricity.
Payback
This depends on what you compare it with, and published payback figures tend to use whichever comparison looks best. All three below use the market gas price.
| 5.2 years | If the old boiler would have run forever, free. £4,375 divided by £838. The simplest and most cautious figure. |
| 1.6 years | If you net off the boiler you'd have bought anyway. Ours is 19 years old and heat-only. A like-for-like replacement is around £3,000 and would have been needed within a few years anyway, so only £1,375 of this is extra spending. |
| ~2.7 years | Our best estimate. The boiler didn't need replacing now, just at some point in the next few years. Money spent now to avoid a cost in year four doesn't cancel it pound for pound, so we've discounted it, which gives roughly this. |
We use the middle figure. We could have quoted 1.6 years, but the boiler we avoided buying was a future cost, and counting it as money already saved inflates the payback. Around 2.7 years is the figure we're comfortable with. On our fixed gas price it would be 3.5, and since that fix ends in March we don't use it as the baseline.
If gas prices change
Keeping the boiler means betting that gas stays cheap, and recently it hasn't. This table runs the same sum across a range of gas prices and real-world efficiencies. Our fix is the top row and today's market price is the second. The rows below show what happens if prices rise the way they have in the last few years.
| Gas price | SCOP 3.0 | SCOP 3.3 | SCOP 3.6 | SCOP 3.8 |
|---|---|---|---|---|
| 6.24p · our fix, until Mar 2027 | £566 | £634 | £688 | £718 |
| 7.39p · market today | £769 | £838 | £892 | £921 |
| 8.00p | £877 | £945 | £999 | £1,029 |
| 9.00p | £1,054 | £1,122 | £1,176 | £1,205 |
| 10.00p | £1,230 | £1,299 | £1,353 | £1,382 |
At 9p gas, which is quite possible over the twenty-year life of a heat pump, the saving is £1,122 a year and the raw payback drops below five years. Each rise in wholesale gas prices makes the boiler look worse, and ours is near the end of its life anyway.
Turning the flow temperature down
Octopus's standard install is commissioned at 57°C. That's quick to set up and means fewer radiator changes, but it costs efficiency for as long as it's left there. The Cosy 9 manages SCOP 3.6 at 50°C, falling towards 3.0 at 65°C. On the market row above, going from SCOP 3.0 to 3.6 is worth about £123 a year, and more if gas goes up. It only takes an afternoon of adjusting and accepting longer run times. We'll do it in spring rather than at commissioning in December, because a cold house in December is a bad way to find out we've turned it down too far.
Why it works here
That £838 isn't what a heat pump would save a typical house. It's what it saves in this one, where the 4,545 kWh of extra winter demand is covered by 10.45 kWp of panels, a 15 kWh battery and an overnight rate under 8p. Put the same unit in a house on a flat tariff with no battery and it would barely beat gas.
The same house with no solar and no battery, running this heat pump and the car on the same tariff, would pay about £3,458 a year for electricity, because none of it could be moved into the cheap window. Without the panels, battery and tariff, the heat pump would only just pay its way. See the tariff for why that window matters so much, and the car for the other half of the electrification bill.
The kit
| Unit | Octopus Cosy 9 monobloc, R290 (propane) refrigerant, with a global warming potential of 3 against 675 for the R32 alternative we were also quoted. |
| Sized for | 7.8 kW surveyed heat loss. The Cosy 9 covers up to 8.7 kW at 50°C flow. |
| Cylinder | 200 L slimline unvented, replacing the old vented tank. The loft header tank comes out with it. |
| Radiators | 9 upgraded to type 22 K2, included in the fixed price. The rest of the house keeps what it has. |
| Underfloor | Already there, already decommissioned from gas, reconnected as part of the install. Underfloor heating works well at low flow temperatures, so it should suit the heat pump better than anything else in the house. |
| Price | £11,875 list, less the £7,500 Boiler Upgrade Scheme grant that Octopus claim on our behalf, leaving £4,375. |
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What we'll know in March
The most important figures on this page are estimates. Heat demand is worked out from gas readings instead of being measured at the radiators. SCOP is the manufacturer's figure, not a reading from our house in a cold spell. The gas price will keep changing, and our fix shields us from that until March 2027. A real January will give us the first two, and the market will decide the third. We'll publish the updated figures either way.